Ternary alkaline-earth pyrosilicate luminescent materials activated with divalent europium

ABSTRACT

Several ternary pyrosilicate compositions of the system MO-MgOSiO2 (M Sr, Ba, Ca) of the general formula M2MgSi2O7 were activated with divalent europium while maintaining the charge balance. Under cathode-ray, and long- and short-wavelength ultraviolet excitation, they show green and/or blue emission depending on the matrix involved. In the phosphors not containing barium, equivalent amounts of magnesium and silicon can be replaced by aluminum and the charge balance maintained. Some embodiments, especially those with high barium content, show good temperature-dependence characteristics, and this phosphor system is useful in fluorescent and high pressure mercury vapor lamps.

United States Patent Datta 1 July 11, 1972 [54] TERNARY ALKALINE-EARTH PYROSILICATE LUMINESCENT MATERIALS ACTIVATED WITH DIVALENT EUROPIUM [72] Inventor: Ranailt Kumar Datta, East Cleveland,

Ohio

[73] Assignee: General Electric Company [22] Filed: April 15, 1969 [21] Appl.No.: 816,273

[52] US. Cl ..252/301.4 F [51] Int. Cl. ..C09k, H54 [58] Field oiSearch ..252/301.4F

[56] References Cited UNITED STATES PATENTS 2,467,810 4/1949 Cassanos et al. ..252/301.4 F 3,544,481 12/1970 Barry ..252/30l.4 F 3,544,482 l2/l970 Barry ..252/301 .4 F 2,297,108 9/1942 McKeag et al... ..252/30l.4 F 3,294,699 12/1966 Lange ..252/30l.4 F 3,503,894 3/1970 Wachtel ..252/30l.4 F

OTHER PUBLICATIONS Blasse et al., Fluorescence of Eu Activated Silicates Philips Research Reports, V. 23, No. 2, April, 1968, pages l89200 pages 189, 190, 191 and 199 Barry, Equilibria and Eu Luminescence of Solidus Phases Bounded by Ba MgS 0 Sr MgS O and Cd MgSi 0 Journal of Electrochemical Society, July 1968 Vol. 115, No. 7, pages 733- 738.

Primary Examiner-Robert D. Edmonds Attomey-Henry P. Truesdell, Frank L. Neuhauser, Oscar B. Waddell, Joseph B. Forman, Melvin M. Goldenberg and John F. McDevitt ABSTRACT Several ternary pyrosilicate compositions of the system MO- MgO-SiO, (M Sr, Ba, Ca) of the general formula M MgSi O were acn'vated with divalent europium while maintaining the charge balance. Under cathode-ray, and longand shortwavelength ultraviolet excitation, they show green and/or blue emission depending on the matrix involved. In the phosphors not containing barium, equivalent amounts of magnesium and silicon can be replaced by aluminum and the charge balance maintained. Some embodiments, especially those with high barium content, show good temperature-dependence characteristics, and this phosphor system is useful in fluorescent and high pressure mercury vapor lamps.

4Clairm,6Drawingfiguros BACKGROUND OF THE INVENTION This invention relates to luminescent materials which convert ultraviolet radiations (both shortand long-wavelength), cathode rays and X-rays into visible radiations. More specifically, the invention relates to ternary alkaline-earth magnesium silicate phosphors.

The unactivated host matrices of the phosphors used in this invention, that is, the ternary magnesium silicates of the alkaline-earth metals, were reported by H. A. Klasens, A. H. Hoekstra and A. P. M. Cox in Ultraviolet Fluorescence of Some Ternary Silicates Activated with Lead in .l. Electrochem. Soc., 104, 93 (Feb, Feb. 1957). Klasen et al. therein activated the ternary silicates with lead (Pb only and reported that the Pb-activated ternary, alkaline-earth silicates show ultraviolet fluorescence under short-wavelength (2,537 A) ultraviolet excitations.

An unactivated compound having the constituents 2SrO.Al by3. SiO is described by Dear in X-Ray Diffraction Data for Silicates, Aluminates and Alumina-Silicates of Strontium, Bull. Va. Polytech. Inst., Vol. L, No. 6, April 1967.

Also, McKeag et al. United States Pat. No. 2,297,108 disclose binary alkaline-earth silicate phosphors, without any magnesium, activated with divalent europium (I-Iu) and several other Eu *activated compounds are known to emit in the blueto green region depending on the matrix.

SUMMARY OF THE INVENTION It is an object of the present invention to provide silicate phosphors having useful and unique luminescent characteristics. A further object is to provide such phosphors whose emission peaks and temperature dependence characteristics can be varied usefully by changing the proportions of alkaline earths in the phosphor.

The present invention in certain of its embodiments provides luminescent materials which are crystalline compounds and have the general composition:

2-x .r 8i-u 2u 2-u 1 wherein:

M is one or more of Ca, Sr, and Ba, A is (Eu, "R,,), R being one or more of Mn and Sn, and

a being from zero to 0.02, x is from a small amount effective for luminescence up to 0.15, y is from zero to 1.0, and when M is Ba alone, y is zero. The activator, A, which includes divalent europium is incorporated into the lattice of the compound.

Preferably, when M is a combination of Sr and Ba, the following relation applies: M is Sr ,,Ba,, and z is between zero and 0.5. I As far as crystallographic structures of the fluorescent materials are concerned, particularly good products obtained by a process according to the present invention exhibit the X- ray diffraction spectra disclosed in the above-cited Klasens et al article of one or more of the following ternary compounds or their crystalline solutions which form in the composition ranges described above:

Ca MgSi 0 $r MgSi 0 Various embodiments of the invention produce good phosphors, although the nature of their emission and temperature-dependence characteristics depend-on the relative proportions of the alkaline-earth metals involved. It is possible that a process described herein may produce certain amounts of one or more of particular species which are largely responsible for the fluorescence of the products of the process. The reaction products, in addition to the preferred embodiments, may contain unreacted starting materials and their disintegration products, and small amounts of other compounds which emit less strongly.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of the emission spectra under 3,650 A excitation of the three phosphors of the invention wherein M is each of Ba, Ca and Sr individually and the Eu content is 0.02 moles per formula unit.

FIG. 2 is a graph of the emission spectra under 3,650 A excitation of five phosphors of the invention wherein M is different proportions of Sr and Ba. The samples shown in this figure were fired at I, C.

FIG. 3 is a graph of the change in total brightness and peak emission as proportions of Ca and Sr are changed in phosphors of the invention.

FIG. 4 is a graph of the emission spectra under 3,650 A excitation of three Al-containing phosphors of the invention.

FIG. 5 is a graph showing the shift in emission peak under 3,650 A excitation of two phosphors of the invention with varying A1 content.

FIG. 6 is a graph of the change in peak emission under 3,650 A excitation of an Al-containing phosphor of the invention with varying proportions of Ca and Sr.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The phosphors can be synthesized by a two-step firing process, provided certain critical precautions are observed.

Appropriate amounts of the alkaline-earth carbonates, silica (or silicic acid) and europium trioxide (Eu 0 are mixed and fired for several hours such as eight hours at a temperature between 900 and l,250 C in air atmosphere; this is followed by grinding and a second firing under reducing conditions for about 2 hours at a temperature between 1,000-1,200 C. If temperatures much below 900 C or much shorter times are used, or if grinding and refiring are omitted, undesirable metastable intermediate silicate phases are formed which have substantial deleterious effects on luminescent characteristics of the resulting material. Formation of these undesirable materials is enhanced if suitable fluxes such as alkaline-earth fluorides are not used. Reducing atmosphere critical to reducing europium to the divalent state can be created by flowing forming gas through or by burning high purity carbon in the semi-airtight reaction chamber.

After the desired heating interval the sample is brought out of the furnace and cooled in the same reducing atmosphere. If a flux is desired, a part of the alkaline-earth may be added as fluoride in the starting material. Theincorporation of such flux materials reduces the temperature and time of heating intervals required to form the ternary silicates of the invention and improves the crystallinity of the fired products. However, the amount of the fluoride in moles should not exceed more than 10 percent of the sum total of the alkaline-earth metals.

The unactivated ternary silicates of the systems MO-MgO- SiO (M Sr, Ba, Ca), such as M MgSi O have a white body color, and they do not respond to cathode rays, X-rays, or longor short-wavelength ultraviolet radiations.

When a small part of the alkaline-earth metal is substituted by Eu, the silicates acquire a white to pale yellow body color and respond very well to cathode rays, X-rays, longand shortwavelength ultraviolet radiations. The nature of the emission and the relative responses are functions of composition. The air fired samples show the typical Eu emission under cathode rays and shortand long-wavelength ultraviolet radiation.

One preferred embodiment, Ba ,.Eu,MgSi O responds very well to radiations with wavelength ranging from 1,800 to 4,000 A, the relative response is a function of the wavelength of the exciting radiation and the Eu content,x. The latter can be varied within wide limits; it has been observed that under 2,537 and 3,650 A excitation, the preferred values of x can range from 0.005 to 0.15, with an optimum about from 0.01 to 0.08. This phosphor emits in a broad band extending from 4,300 A to 7,000 A with a peak at 5,050 A, as shown in FIG. 1. The band shape is essentially independent of the activator (I-Iu) concentration and the wavelength of the exciting source. The total brightness of Ba MgEu Si O, measured through an eye sensitivity filter is about 90 percent of that of commercial Zn SiO :Mn under 2,537 A excitation. This phosphor shows good temperature stability: at 250 C the brightness under 3,650 and 2,537 A excitations are about 65 and 50 percent, respectively, of the maximum brightness under the corresponding radiation which occurs at room tem perature.

Another preferred embodiment, Sr ,.Eu MgSi O,, when excited by cathode rays, X-rays, longand short-wavelength ultraviolet radiations, emits in a broad band extending from 4,200 A to 6,000 A with a peak at 4,700 A, as shown in FIG. 1. Like the Barium Analogue, intensity of emission of Sr -MgE u ,Si,O under various excitations is a function of the concentration x of Eu. The concentration can be varied within wide limits, preferably from 0.005 to 0.15, with an optimum about from 0.01 to 0.08.

Depending on the temperature of reaction, varying amounts of Ba Eu ,.MgSi,O, can be dissolved in the Sr ,,.Eu,,.MgSi O lattice: at 1,200 C, about 70 mole percent of Ba Eu MgSi O can be dissolved in the sr, ,,Eu,, M si o, lattice causing a shift in the peak emission of the latter embodiment toward shorter wavelength. FIG. 2 shows the emission spectra of several samples of the system Sr, Ba,Eu ,MgSi O-,. These samples were prepared at l,l C. The peak emission of the embodiment, Sr ,Ba,Eu MgSi O excited under 3,650 A radiation, shifts from 4,700 A to 4,350 A as 2 increases from 0 to 0.98, as shown in FIG. 2. With further increase in z, the maximum possible solubility of Ba in the Sr, Eu Mgsi oflattice at 1,100 C is reached and the barium analogue of the embodiment with its broad emission peaking at 5,050 A appears as an additional phase. Incorporation of barium in the Sr Eu MgSi O-, lattice also improves the temperature-brightness relationship of the embodiment: under 3,650 A excitation at 50 C, Sr Eu MgSi O shows 50 percent of its maximum brightness which occurs at room temperature, whereas, when the composition of the embodiment is changed to Sr Ba Eu MgSi O the brightness at 50 C is about 65 percent of the maximum brightness which again occurs at room temperature. Solubility of Sr Eu MgSi- O in the Ba Eu Mgsi O lattice at l,200 C is negligible.

In Ca Sr,- Eu,, .,,M si,o,, as z is increased from 0 to 1.98, the peak is shifted from 5,280 A to 4,700 A under 3,650 A excitation. The change in total brightness and peak position of these phosphors with change in z is shown in FIG. 3, with a maximum brightness at z of about 0.9.

The emission spectra of Ca Eu AhSiO Sr, Eu Al SiO,, and Sr, Ba Eu Al SiO-, are shown in FIG. 4, with peaks respectively near 4,450, 4,800 and 5,250 A. These materials all have an off-white body color.

The shift in peak emission of M, Eu Mg, ,,Al ,,Si ,,O-, wherein M is either Sr or Ca is shown in FIG. 5. Such a luminescent compound is not formed when M is Ba. However, small amounts of Ba can be incorporated in the lattice and substituted for Sr when M is Sr.

The change in peak emission of Ca, ,'Sr,'Eu ,Al SiO with the varying ofz is shown in FIG. 6.

In certain preferred embodiments, it has been found that at least 0.005 moles per formula unit Eu, and up to 0.08 moles Eu, are desirable. Preferred ranges of y are between 0.005 and 0.5 moles per formula unit, equivalent to 0.01 to 1.0 moles Al. Furthermore, when y is zero and M is (Sr, ,Ba,), z is preferably between zero and 0.75.

EXAMPLE I A batch consisting of the following ingredients was mixed together and tired at l,l00 C for several hours such as 8 hours in air:

8.79 gm of SrCO 2.835 gm ofBasic MgCO,

0.1056 gm ofEu o 3.96 gm of Hydrated silica The basic MgCO used in this application has 42.67 percent MgO by weight. The hydrated silica used has 98.6 percent siO by weight. The cooled sample was ground and refired at l,050 C in reducing atmosphere for l to 2 hours. Straight forming gas 5 percent H balance N passed at a rate of 1,200 cc/min through the air-tight silica-glass reaction chamber was adequate to reduce Eu *to Eu. The sample was brought out of the furnace and cooled in the same reducing atmosphere. The final product was a white powder which on excitation under cathode rays or ultraviolet radiation showed a broad emission spectrum extending from 4,200 A to 6,000 A with a peak at 4,700 A.

EXAMPLE II To demonstrate the use of BaF flux, the following ingredients were mixed together and treated as described in Example I:

7.54 gm of BaCO 1.89 gm of Basic MgCO 2.64 gm of Hydrated silica 0.1232 gm ofEu- O 0.175 gm ofBaF The final product was a pale yellow powder which showed well crystallinity under X-ray diffraction analysis. Under cathode rays and ultraviolet radiations, it showed a broad emission spectrum extending from 4,300 A to 7,000 A with a peak at 5,050 A. The total brightness of the sample with an eye sensitivity filter is about 75 percent of that of commercial Zn SiO Mn.

EXAMPLE III This slight variation on Example II was made with the ingredients expressed in moles as follows:

BaCO l.93 moles BaF 0.05 moles Eu O 0.01 moles MgCO LO moles SiO- 2.0 moles The results were essentially the same as stated for Example II.

EXAMPLE IV m Mg m 2 1 The following ingredients were mixed together and fired in air atmosphere at 1,l00 C for several hours:

4.44 gm of SrCO 5.79 gm of BaCO 2.835 gm of Basic MgCO 3.96 gm of Hydrated silica 0.1056 gm of Eu O The sample was cooled, ground and retired at 1,050 C in a reducing atmosphere for l to 2 hours. The reducing atmosphere was created by burning pure carbon blocks inside the semi-airtight reaction vessel where the sample was placed in a separate container to prevent any physical contact with carbon. The whole reaction vessel containing the sample and the carbon block was taken out of the furnace and cooled. The phosphor sample obtained was of pale yellow color; when excited by cathode rays of ultraviolet radiation, it emitted in a broad band extending from 4,200 A to 6,000 A with a peak at 4,350 A.

EXAMPLE v EXAMPLE Vl n. om om 2 'l The batch ingredients used were:

4.395 gm SrCO 2.99 gm CaCO 0.1056 gm Eu o 1.98 gm Hydrated silica The ingredients were mixed and heated as in Example I. The final product, a pale yellow polycrystalline material, showed a broad emission with a peak at about 5,100 A under 2,537 and 3,650 A excitation.

What I claim is:

l. Luminescent material consisting essentially of a crystalline compound having the general composition:

a-q zgl-u zv a-u 7 wherein M is one or more of Ca, Sr, and Ba,

A is (Eu R R being Sn, and a being from zero to x is from a small amount effective for luminescence up to y is from 0.005 to 0.5.

2. Luminescent material according to the formula:

wherein M is one or more of Ca and Sr and x is from a small amount effective for luminescence up to 3. Luminescent material according to claim 2 in which M is Sr.

4. Luminescent material according to claim 2 in which M is Ca. 

2. Luminescent material according to the formula: M2 xEuxAl2SiO7 wherein M is one or more of Ca and Sr and x is from a small amount effective for luminescence up to 0.15.
 3. Luminescent material according to claim 2 in which M is Sr.
 4. Luminescent material according to claim 2 in which M is Ca. 